Heterogeneous Networks do not promote cooperation when humans play a Prisoner’s Dilemma

It is not fully understood why we cooperate with strangers on a daily basis. In an increasingly globalized world, where interaction networks and relationships between individuals are becoming more complex, different hypotheses have been put forward to explain the foundations of human cooperation on a large scale and to account for the true motivations that are behind this phenomenon. In this context, population structure has been suggested to foster cooperation in social dilemmas, but theoretical studies of this mechanism have yielded contradictory results so far; additionally, the issue lacks a proper experimental test in large systems. We have performed the largest experiments to date with humans playing a spatial Prisoner’s Dilemma on a lattice and a scale-free network (1,229 subjects).

We observed that the level of cooperation reached in both networks is the same, comparable with the level of cooperation of smaller networks or unstructured populations. We have also found that subjects respond to the cooperation that they observe in a reciprocal manner, being more likely to cooperate if, in the previous round, many of their neighbors and themselves did so, which implies that humans do not consider neighbors’ payoffs when making their decisions in this dilemma but only their actions. Our results, which are in agreement with recent theoretical predictions based on this behavioral rule, suggest that population structure has little relevance as a cooperation promoter or inhibitor among humans.

Behavioral transition with age in social dilemmas:From reciprocal youth to persistent response in adulthood

While human societies are extraordinarily cooperative in comparison with other social species, the question of why we cooperate with unrelated individuals remains open. Although much experimental studies have been performed to elucidate several questions related to the emergence and evolution of cooperation in humans, little has been done regarding how cooperativeness changes across our lifetime. This work reports results of a lab-in-the-field experiment with people of different ages in a social dilemma. We found that the average amount of cooperativeness is independent of age except for the elderly, who cooperated more, and a behavioral transition from reciprocal, but more volatile

behavior to more persistent actions towards the end of adolescence. Although all ages reacted to the cooperation received in the previous round, young teenagers mostly responded to what they saw in their neighborhood regardless of their previous actions. Decisions then became more predictable through midlife, when the act of cooperating or not was more likely to be repeated. Our results show that mechanisms such as reciprocity, which is based on reacting to previous actions, may promote cooperation in general, but its influence can be hindered by the fluctuating behavior in the case of children.

Dynamic Networks & Reputation

Cooperativeness is a defining feature of human nature. Theoreticians have suggested several mechanisms to explain this ubiquitous phenomenon, including reciprocity, reputation, and punishment, but the problem is still unsolved. Here we show, through experiments conducted with groups of people playing an iterated Prisoner’s Dilemma on a dynamic network, that it is reputation what really fosters cooperation. While this mechanism has already been observed in unstructured populations, we find that it acts equally when interactions are given by a network that players

can reconfigure dynamically. Furthermore, our observations reveal that memory also drives the network formation process, and cooperators assort more, with longer link lifetimes, the longer the past actions record. Our analysis demonstrates, for the first time, that reputation can be very well quantified as a weighted mean of the fractions of past cooperative acts and the last action performed. This finding has potential applications in collaborative systems and e-commerce.